Technologies to Watch: Digital Flowmeters

Jan. 30, 2008

By Matt Migliore The Foxboro CFT50 digital Coriolis flowmeter features a patented software-based system that is designed to prevent erratic liquid/gas flowtube vibrations that cause measurement failures, thus eliminating

By Matt Migliore

The Foxboro CFT50 digital Coriolis flowmeter features a patented software-based system that is designed to prevent erratic liquid/gas flowtube vibrations that cause measurement failures, thus eliminating air-induced interruptions or stoppages during two-phase events.

The move from analog to digital is nothing new when it comes to fluid handling technology, but this trend is particularly evident in the flowmeter space. Here, advances in microelectronics have enabled digital flowmeter suppliers to reduce the amount of time it takes to arrive at a measurement by, in some cases, hundreds of percent. For example, Coriolis flowmeters, which were once hampered by response times of about 0.25 seconds, are now capable of responding to changes in flow within milliseconds. Thus, digital flowmeter technologies are attracting more and more interest from end-users.

One of the key advantages digital flowmeters have gained as a result of increased processing power and faster response times is the ability to support small volume prover testing. Small volume provers are commonly used as standards in the field for testing the accuracy of flow measurement devices. As such, compatibility with small volume provers has opened the door for some digital flowmeter technologies to receive key industry approvals, which have been a tremendous boon in terms of spurring adoption rates.

The newest generation of Coriolis flowmeters are capable of serving as a condition monitoring system to alert users about and quantify two-phase flow conditions, while at the same time enabling a flow measurement accurate enough to keep the process in control.

For example, the American Gas Association’s AGA-9 report, which first detailed criteria for the use of ultrasonic flowmeters in custody transfer of natural gas in June 1998, has generated such rapid growth in demand for ultrasonic technology that this flowmeter segment has emerged from what was once considered a niche solution to become arguably today’s most popular flowmeter technology type. Likewise, the American Petroleum Institute’s recommendation for Coriolis technology has generated significant interest in this flowmeter type, while a new API draft standard recommending vortex flowmeters for custody transfer of petroleum figures to have a similar effect in this category.

Here an old technology Coriolis meter and a new one are compared in series in a batching operation. The digital flowmeter is providing both a density (effectively a void fraction) measurement and a mass flow measurement shown in black, while the gray trace shows the older technology flowmeter. As the meter is filling during the first several seconds of the batch, as shown by the density measurement, the digital Coriolis meter is metering the flow immediately. Whereas the older Coriolis meter is essentially out of commission and missing measurement for more than 10 seconds. The area between the black and gray curves represents unaccounted for product.

From a technology perspective, Coriolis flowmeters offer a solid example of the sea-change currently under way in the flowmeter space due to advances in microelectronics. With the ability to respond to flow changes in the milliseconds range, Coriolis flowmeters can now support two-phase flow applications, where a mixture of liquids and gases is present in the flow stream — a measurement that has historically been problematic for most flowmeter types. In addition, Coriolis meters are now also capable of high performance in batching applications, where the process begins with an empty pipe.

According to Wade Mattar, flow specialist for Invensys Process Systems/Foxboro Company (, while Coriolis flowmeters have always been able to measure liquids and gases, they, along with most other flowmeter types, have traditionally struggled with measurements in the transition phase. Now, he says, with digital control and high-speed processing capability, Coriolis flowmeters have the firepower to provide accurate measurements in just about any flow condition.

And such is the goal of technologists working to develop the latest generation of flowmeter solutions, says Mattar, noting "The whole objective is to not have to ask a user of a meter to make a change to their process to get a measurement."

Beyond processing speed, advances in microelectronics have also boosted the memory capacity of digital flowmeters many times over, thus providing access to a plethora of diagnostic information about the performance of flow measurement systems.

In applications where diagnostics are employed, Mattar says it is important for the supplier to work with the user to determine how to move information through the appropriate channels. "You have to be careful to provide the user only the diagnostics they can act on," he says. This isn’t to say that a given diagnostic is useless, he says, it just needs to be delivered in the proper way. "These diagnostics can be there," says Mattar, "but they shouldn’t trigger alarms and flashing screens if they don’t require an action by the user."

In discussing the value traditional diagnostics bring to the end-user, Mattar says, "Lets face it, users want instruments that are reliable and don’t fail, but the reality is they do fail, for a variety of reasons, and the user needs to be alerted in a timely fashion when they do. However, for obvious reasons, the user would get much more value in being alerted before they fail.,"

As such, Mattar believes the future is much brighter for predictive diagnostics in the flowmeter segment. Whereas traditional diagnostics are typically used to trigger alarms for failure events and are more reactive in nature, predictive diagnostics allow users to be proactive in managing their flow measurement systems. For example, predictive diagnostics allow users to set certain performance thresholds that would allow them to organize maintenance schedules in a logical fashion, thus limiting the likelihood of unplanned shutdowns.

Going a step farther, Mattar is also bullish on the future for condition monitoring technologies, where end-users aren’t just monitoring performance on a device-by-device basis, but are integrating this information to maximize the efficiency of the larger process. "We’ve got these devices with all of this intelligence and horsepower," he says, "why can’t they also be used as window into your monitoring system to tell you more about your process?"

Going forward, Mattar says users can look for flowmeter technology to continue to evolve to support new applications. For example, he says one area where flowmetering can improve is in oil wellhead applications, which currently require expensive separators to arrive at a flow measurement. Mattar says it would be particularly useful if a flowmeter could handle wellhead applications without the assistance of a separator. Ultimately, he says, "One area we are concentrating on is trying to make measurements where measurements have either never been made before or have been made but in a less than optimal way."

Matt Migliore is the editor of Flow Control magazine. He can be reached at [email protected] or 610 828-1711.

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